TWO-TRANSISTOR MODEL OF A THYRISTOR

The principle of thyristor operation can be explained with the use of its two-transistor model (or two-transistor analogy). Fig. 4.15 (a) shows schematic diagram of a thyristor. From this figure, two-transistor model is obtained by bisecting the two middle layers, along the dotted line, in two separate halves as shown in Fig. 4.15 (b). In this figure, junctions J₁ – J₂ and J₂ -J3 can be considered to constitute pnp and npn transistors separately. The circuit representation of the two-transistor model of a thyristor is shown in Fig. 4.15 (c).

In the off-state of a transistor, collector current I_c is related to emitter current I_E as

I_C = αI_E + I_CBO

where α is the common-base current gain and I_CB0 is the common-base leakage current of collector-base junction of a transistor.

For transistor Q₁ in Fig. 4.15 (c), emitter current I_E = anode current I_a and I_C = collector current I_C1. Therefore, for Q₁

I_C1 = α₁ I_a +  I_CBO1 ……..(4.3)

where      α₁ = common-base current gain of Q₁

and          I_{CBO1  =} common-base leakage current of Q₁

Similarly, for transistor Q₂, the collector current I_C2 is given by

I_C2 = α₂ I_k +  I_CBO2 …(4.4)

where      α₂ – common-base current gain of Q₂,I_CBO2 =common-base leakage current of Q₂ and

I_k = emitter current of Q₂.

The sum of two collector currents given by Eqs. (4.3) and (4.4) is equal to the external circuit current   I_α entering at anode terminal A.

There fore   I_a = I_{C1 +} I_C2

I_{a =} α₁ I_{a +} I_CBO1+ α₂ I_k +  I_CBO2 …(4.5)

When gate current is applied, then I_k = I_a + I_g . Substituting this value of I_k in Eq. (4.5) gives

I_{a =} α₁ I_{a +} I_CBO1+ α₂ (I_a + I_g ) +  I_CBO2

_or

I_{a =} α₂ I_{g +} I_{CBO1 +} I_CBO2 /[1-( α₁₊ α₂)]

For a silicon transistor, current gain α is very low at low emitter current. With an increase in emitter current, a builds up rapidly as shown in Fig. 4.16. With gate current I_g = 0 and with thyristor forward biased,( α₁₊ α₂)is very low as per Eq (4.6) and forward leakage current somewhat more than I_{CBO1 +} I_CBO2 flows. If, by some means, the emitter current of two component transistors can be increased so that α₁₊ α₂ approaches unity, then as per Eq. (4.6) I_a would tend to become infinity thereby turning-on the device. Actually, external load limits the anode current to a safe value after the thyristor begins conduction. The methods of turning-on a thyristor, in fact, are the methods of making α₁₊ α₂ to approach unity. These 0.25 various mechanisms for turning-on a thyristor are now discussed below :

(i)         GATE Triggering : With anode positive with respect to cathode and with gate current I_g = 0, Eq. (4.6) shows that anode current, equal to the forward leakage current, is somewhat more than  I_{CBO1 +} I_CBO2,Under these conditions, the device is in the forward blocking state.

Now a sufficient gate-drive current between gate and cathode of the transistor is applied. This gate-drive current is equal to base current I_{B2 =} I_g and emitter current I_k of transistor Q₂. With the establishment of emitter current I_k of Q₂, current gain α₂ of Q₂ increases and base current I_B2 causes the existence of collector current I_C2 = β₂I_B2 = β₂ I_g. This amplified current I_C2 serves as the base current I_B1 of transistor Q₁ With the flow of I_B1 collector current I_C1 = β₁ I_B1 = β₁ β₂ I_g of Q₁ comes into existence. Currents I_B1 and I_C1 lead to the establishment of emitter current I_a of Q₁ and this causes current gain α₁ to rise as desired. Now current I_g + I_CI = (1 + β₁ β₂) I_g acts as the base current of Q₂ and therefore its emitter current I_k = I_CI + I_g With the rise in emitter current I_k α₂ of Q₂ increases and this further causes I_C2 = P₂ (1 + β₁ β₂) I_g to rise. As amplified collector current I_C2 is equal to the base current of Q₁ current gain α₁ eventually rises further. There is thus established a regenerative action internal to the device. This regenerative or positive feedback effect causes α₁₊ α₂ to grow towards unity. As a consequence, anode current begins to grow towards a larger value limited only by load impedance external to the device. When regeneration has grown sufficiently, gate current can be withdrawn. Even after I_g is removed, regeneration continues. This characteristic of the thyristor makes it suitable for pulse triggering. Note that thyristor is a latching device

After thyristor is turned on, all the four layers are filled with carriers and all junctions are forward biased. Under these conditions, thyristor has very low impedance and is in the forward on-state.

(ii) Forward-voltage triggering : If the forward anode to cathode voltage is increased, the collector to emitter voltages of both the transistors are also increased. As a result, the leakage current at the middle junction J₂ of thyristor increases, which is also the collector current of Q₂ as well as Q₁ With increase in collector currents I_C1 and I_C2 due to avalanche effect, the emitter currents of the two transistors also increase causing α₁₊ α₂ to approach unity. This leads to switching action of the device due to regenerative action. The forward-voltage triggering for turning-on a thyristor may be destructive and should therefore be avoided.

(iii) dv/dt triggering : The reversed biased junction J₂ behaves like a capacitor because of the space-charge present there. Let the capacitance of this junction be Cj. For any capacitor, i = C dv/dt.In case it is assumed that entire forward voltage v_a appears across reverse biased junction J₂ then charging current across the junction is given by

i = Cj dv_a /dt

This charging or displacement current across junction J₂ is collector currents of Q₂ and Q₁ Currents I_C2, I_C1 will induce emitter current in Q₂, Q₁ In case rate of rise of anode voltage is large, the emitter currents will be large and as a result, α₁₊ α₂ will approach unity leading to eventual switching action of the thyristor.

(iid Temperature triggering : At high temperature, the forward leakage current across junction J₂ rises. This leakage current serves as the collector junction current of the component transistors Q₁ and Q₂. Therefore, an increase in leakage current I_CI, I_C2 leads to an increase in the emitter currents of Q_l Q₂. As a result, (α₁₊ α₂) approaches unity. Consequently, switching action of thyristor takes place.

(v) Light triggering : When light is thrown on silicon, the electron-hole pairs increase. In the forward-biased thyristor, leakage current across J₂ increases which eventually increases α₁₊ α₂ to unity as explained before and switching action of thyristor occurs.

As stated before, gate-triggering is the most common method for turning-on a thyristor. Light-triggered thyristors are used in HVDC applications.

The operational differences between thyristor-family and transistor family of devices may now be summarised as under :

i) Once a thyristor is turned on by a gate signal, it remains latched in on-state due to internal regenerative action. However, a transistor must be given a continuous base signal to remain in on-state.

ii) In order to turn-off a thyristor, a reverse voltage must be applied across its anode-cathode terminals. However, a transistor turns off when its base signal is removed.

Written by arjun on April 14th, 2009 with no comments.
Read more articles on Power Electronics and Thyristor.

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